(a) Field of the Invention
The present invention relates to an apparatus for growing, by utilizing a floating zone, a GaAs compound semiconductor single crystal which contains As having a high vapor pressure as a constituent component of the crystal.
(b) Description of the Prior Art
As is well known, a GaAs single crystal which is a compound semiconductor is used as a substrate in fabricating a device such as a field effect transistor, an SIT (static induction transistor), IC, millimeter ware diode, Tunnett diode, light-emitting diode, or a laser diode, and concurrently researchers are making a steady progress with a great expectation for the development of good GaAs single crystals to serve as a material for fabricating such semiconductor devices as transistors and semiconductor integrated circuits which are operated at high speeds, from the viewpoint that the electron mobility of a GaAs compound semiconductor is 8500 cm.sup.2 /V.multidot.sec which is more than five times greater than that of silicon.
As a matter of course, as the GaAs single crystal which is used as the substrate for the fabrication of such semiconductor devices as mentioned above, the crystal is desired to have an excellent crystal quality.
In the past, GaAs single crystals for use as substrates have been obtained by a single crystal growth method such as the horizontal Bridgman method or the LEC (Liquid Encapsulated Czochralski) method which uses a B.sub.2 O.sub.3 layer for encapsulation of the growth zone. The former method has disadvantages such that, because it uses a quartz tube for the growth, it is difficult to obtain a single crystal of a large diameter, and that, since it is usual to grow a crystal in the (111) orientation, there are the inconveniences that (1) in order to obtain a substrate having a (100) surface which is frequently used in semiconductor devices, the crystal has to be cut not perpendicular to the (111) orientation but obliquely relative to the surface of the crystal; and (2) although it is usual to obtain a semi-insulating or highly resistive substrate by an inclusion of an impurity which is Cr or O or both, it is necessary that such an impurity or impurities must be introduced at a high temperature of 800.degree. C. or higher, so that the inclusion of such an impurity cannot be accomplished in a stable manner. The latter method can produce a crystal of a large diameter in the (100) surface. However, since this method uses a B.sub.2 O.sub.3 layer as a sealing material, the temperature gradient between the GaAs melt and the seed crystal is steep, being 100.degree..about.300.degree. C./cm, and accordingly there is the disadvantage that the crystal which is grown easily tends to have a very large etch pits density (EPD) and develop such defects as lattice dislocation. Other than those conventional methods which have been mentioned above, there is performed Czochralski's pulling method. This known method also has similar drawbacks to those mentioned above.
There are, in fact, more important problems as will be described hereunder.
Unlike such a crystal such as silicon crystal, a GaAs compound semiconductor is a crystal which is produced by compounding two different elements. Besides, the III column element Ga has a vapor pressure which is substantially different from the vapor pressure of the V column element As. The vapor pressure of As is by far greater than that of Ga. Accordingly, the grown GaAs crystal exhibits a considerable deviation from stoichiometory. It has been usual in the growth of a GaAs crystal by conventional manners that As atoms show a shortage within the crystal so that such defects as As vacancies or lattice dislocation, or the defect representing a combination of As vacancies and impurity atoms occur.
Recent research has revealed that the deviation from stoichiometry of the grown crystal imparts a very great effect upon the characteristics or the life of the devices formed with such crystals having a deviation of stoichiometry. The present inventor, in order to solve the above-mentioned problems in the technical field of epitaxial growth, earlier proposed in Japanese Patent Application No. Sho 57-118916 a method for supplying As vapor throughout the growth process, with successful results. However, in the field of growing a single crystal in the abovesaid earlier proposed method, there was not given a consideration to a growth apparatus which is constructed to avoid such deviation from stoichiometry of the grown crystal.